The approaches described in this section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
In computer networks such as the Internet, packets of data are sent from a source to a destination via a network of elements including links (communication paths such as telephone or optical lines) and nodes (for example, routers directing the packet along one or more of a plurality of links connected to it) according to one of various routing protocols.
One routing protocol used, for example, in the internet is Border Gateway Protocol (BGP). BGP is used to route data between routing domains such as autonomous systems (AS) comprising networks under a common administrator and sharing a common routing policy. BGP routers exchange full routing information during a connection session for example using Transmission Control Protocol (TCP) allowing inter-autonomous system routing. The information exchanged includes various attributes including a next-hop attribute. For example where a BGP router advertises a connection to a network, for example in a form of an IP address prefix, the next-hop attribute comprises the IP address used to reach the BGP router.
Edge or border BGP routers in a first AS (ASBRs) communicate with eBGP peers in a second AS via exterior BGP (eBGP). In addition BGP routers within an AS exchange reachability information using interior BGP (iBGP). As a very large number of routes may be advertised in this manner an additional network component comprising a route reflector is commonly provided which sets up a session with each BGP router and distributes reachability information to each other BGP router.
The border routers in respective AS's can advertise to one another, using eBGP, the prefixes (network destinations) reachable from them, the advertisements carrying information such as AS-path, indicating the AS's through which the route advertisement has passed including the AS in which the advertising border router itself is located, and a BGP Community attribute indicating the manner in which the advertisement is to be propagated. For example if an eBGP advertisement is received with Community attribute No-Advertise, then the border router receiving the advertisement does not advertise the route information to any of its peers, including other routers in its AS. When the routes are advertised internally using iBGP, additional information such as a local preference and a nexthop field are included. The local preference attribute sets a preference value to use of that particular route for example for a given set of prefixes such that where more than one route is available to other border routers in the AS they will select the route with the highest local preference. The next-hop attribute provides the IP address used for the link between the border router in the AS and its eBGP peer.
To reduce the amount of iBGP messages further, route reflectors may only advertise the best path for a given destination to all border routers in an AS. Accordingly all border routers will forward traffic for a given destination to the border router identified in the best path advertisement. Forwarding of packets within the AS may then simply use Interior Gateway Protocol (IGP) as described in more detail below where the IGP forwarding table will ensure that packets destined for the eventual destination will be forwarded within the AS towards the appropriate border router. Alternatively an ingress border router receiving incoming packets may tunnel the packets to the appropriate egress border router, that is, encapsulate the packets to a destination egress border router for example using IP or MPLS tunnels. The packets are then decapsulated at the egress border router and forwarded according to the packet destination header.
BGP is capable of supporting multiple address types for example internet protocol version 4 (IPv4), internet protocol version 6 (IPv6) and so forth, and each type of address is identified using an address family identifier (AFI) and a subsequent address family identifier (SAFI). The destinations reachable via a BGP route, for example the network components whose IP addresses are represented by one IP prefix, are referred to as the network layer reachability information (NLRI) in BGP.
Within each AS the routing protocol typically comprises an interior gateway protocol (IGP) for example a link state protocol such as open shortest path first (OSPF) or intermediate system-intermediate system (IS-IS).
The link state protocol relies on a routing algorithm resident at each node. Each node on the network advertises, throughout the network, links to neighboring nodes and provides a cost associated with each link, which can be based on any appropriate metric such as link bandwidth or delay and is typically expressed as an integer value. A link may have an asymmetric cost, that is, the cost in the direction AB along a link may be different from the cost in a direction BA. Based on the advertised information in the form of a link state packet (LSP) each node constructs a link state database (LSDB), which is a map of the entire network topology, and from that constructs generally a single optimum route to each available node based on an appropriate algorithm such as, for example, a shortest path first (SPF) algorithm. As a result a “spanning tree” (SPT) is constructed, rooted at the node and showing an optimum path including intermediate nodes to each available destination node. The results of the SPF are stored in a routing information base (RIB) and based on these results the forwarding information base (FIB) or forwarding table is updated to control forwarding of packets appropriately. When there is a network change an LSP representing the change is flooded through the network by each node adjacent the change, each node receiving the LSP sending it to each adjacent node.
As a result, when a data packet for a destination node arrives at a node the node identifies the optimum route to that destination and forwards the packet to the next node along that route. The next node repeats this step and so forth.
It is important to minimize packet loss in the case of network component failure, both intra-domain (eg IGP) and inter-domain (eg BGP). For example in the case of intra domain link failure ISP's use various techniques to react quickly to the failure while convergence is taking place including handling of the failures by other layers or implementing fast reroute techniques for example of the type described in co-pending patent application Ser. No. 10/340,371, filed 9 Jan. 2003, entitled “Method and Apparatus for Constructing a Backup Route in a Data Communications Network” of Kevin Miles et al., (“Miles et al.”), the entire contents of which are incorporated by reference for all purposes as if fully set forth herein.
In the case of inter-domain failure, for example failure of peering links between AS's, convergence can take several seconds. In one mode of operation, in these circumstances, a BGP router attached to a failed eBGP peering link advertises a new LSP without the destination served by the failed link together with an iBGP withdraw message indicating that the destinations are not reachable. A solution to the problem of inter-domain failure has been described in co-pending patent application Ser. No. 11/254,469, filed Oct. 20, 2005, entitled “A Method of Constructing a Backup Path in an Autonomous System” of Clarence Filsfils et al (“Filsfils et al”), the entire contents of which are incorporated by reference for all purposes as if fully set forth herein. As described in Filsfils et al, in the case where an AS has links with multiple AS's serving respective sets of destinations or prefixes, a backup path is constructed in the case of failure of a link to a respective one of the multiple AS's by identifying alternate links serving the same set of destinations, providing per-prefix route protection.
Currently, when a prefix is advertised in iBGP, the routers contained in the AS must derive the reliability of (redundancy in) the external connection to each prefix themselves from the number of adverts they receive displaying connectivity to said prefix. This leads to a computational overhead which slows down iBGP convergence.